1. Field
The present invention relates to a power cable with a termination connection box, and more particularly, to a power cable with a termination connection box, in which the occurrence of heterojunction interfaces is reduced to prevent accumulation of space charges so as to mitigate an electric field when the power cable is coupled to the termination connection box, and which can be reduced in size by reducing the size of the termination connection box.
2. Description of the Related Art
In general, a power cable is installed under the ground, on the ground, or at the bottom of a sea to supply power to a desired location using a conductor which supplies power. In the power cable, it is very important to insulate the conductor. To this end, an insulating layer for insulating the conductor is formed of cross-linked polyethylene (XLPE) or the like or is formed by winding insulating paper or the like.
A so-called ‘paper insulation cable’ including an insulating layer formed by winding the insulating paper may be classified as an oil-filled (OF) cable or a mass-impregnated (MI) cable according to the viscosity of insulating oil used. In the OF cable, the insulating paper is impregnated with relatively low-viscosity insulating oil. In contrast, in the MI cable, the insulating paper is impregnated with relatively high-viscosity insulating oil. In the OF cable, the insulating paper is impregnated with the relatively low-viscosity insulating oil to cause the insulating oil to flow within the cable. In the MI cable, the relatively high-viscosity insulating oil is used and thus hardly flows in the insulating paper.
When the temperature of the MI cable increases to a predetermined temperature or more, the insulating oil may flow to the outside of the insulating layer. However, even if the temperature of the MI cable decreases, the insulating oil does not return to the original position thereof and thus a portion of the insulating layer from which the insulating oil escapes becomes a void. The dielectric strength of the insulating layer may be degraded due to the void. In order to solve this problem, Japanese Patent Application No. 1994-306654 (hereinafter referred to as ‘Document 1’) employs composite insulating paper formed of plastic and kraft paper when an insulating layer is formed of insulating paper impregnated with high-viscosity insulating oil. Document 1 discloses a three-layer structure in which kraft paper is stacked on the top and bottom of a plastic layer. In Document 1, voids caused by escape of the insulating oil are reduced by suppressing movement of the insulating oil due to thermal expansion of the plastic layer, thereby preventing dielectric breakdown from occurring.
Joint boxes are coupled to the power cable at intervals of several hundreds of meters or several tens of kilometers, and a termination connection box is coupled to an end of the power cable. The termination connection box may be classified as an air-termination connection box, a gas-termination connection box, or an oil-termination connection box according to a state in which an exposed end portion of a conductor of the cable is connected. However, an insulating layer of the power cable is exposed inside the joint boxes or the termination connection box. In this case, when the insulting layer is formed of a plurality of types of materials as in Document 1, a plurality of heterojunction interfaces may be generated when the power cable is coupled to the joint boxes or the termination connection box. When the plurality of heterojunction interfaces are generated, space charges may be accumulated and thus an electric field may be concentrated.
WO 2012-116712 (hereinafter referred to as ‘Document 2’) discloses a method of joining high-voltage paper insulation cables. Document 2 discloses a method of exposing conductors included in the cables by removing protective layers and outer semiconducting layers, cutting insulating layers in a direction substantially perpendicular to a lengthwise direction thereof, and removing inner semiconducting layers, and then coupling the conductors to each other, and a method of forming the inner semiconducting layer, the insulating layer, and the outer semiconducting layer. However, Document 2 relates to joining of the paper insulation cables but does not particularly describe materials of the insulating layer and a reinforcing insulating layer. Thus, problems caused by heterojunction interfaces as described above and a structure for preventing the occurrence of heterojunction interfaces are not suggested in Document 2.
Japanese Patent Application No. 1992-317947 (hereinafter referred to as ‘Document 3’) discloses a joint box for a paper insulation cable, in which when a reinforcing insulating layer is provided, the reinforcing insulating layer is formed of the same insulating paper as an insulating layer of the cable or is formed of insulating paper having lower permittivity than that of the insulating layer of the cable. However, Document 3 simply discloses a structure in which the insulating paper having lower permittivity than that of the insulating layer of the cable is provided such that high voltage is applied to the insulating paper to decrease electric stress.
Various examples of a termination connection box for the paper insulation cable are disclosed in Japanese Patent Application No. 1995-242989 (hereinafter referred to as ‘Document 4’), Japanese Patent Application No. 1997-041178 (hereinafter referred to as ‘Document 5’), Japanese Patent Application No. 1995-022086 (hereinafter referred to as ‘Document 6’), and Japanese Patent Application No. 1995-022085 (hereinafter referred to as ‘Document 7’).
Document 4 simply discloses a termination structure for a MI cable, in which an insulating layer is formed of a compound such as magnesium oxide (MgO).
Document 5 discloses a termination structure for a MI cable, in which a reinforcing insulating layer and an electric-field mitigation cone are formed by covering an exposed end portion of the cable with a plurality of pieces of paper having different thicknesses.
Documents 6 and 7 disclose termination connection boxes for a MI cable, in which a clearance is formed between a cable core and a condenser core, through which insulating oil flows.
Accordingly, although Documents 4 to 7 disclose termination connection boxes for the MI cable but do not suggest problems caused by heterojunction interfaces between the cable and the termination connection boxes and a structure preventing the heterojunction interfaces from being formed.